Femoral slideway

ABSTRACT

A femoral slideway and a femoral slideway/femur-size template combination is employed to create an installed knee prosthesis that creates a reduced amount of stress in the collateral ligaments relative to previous prostheses. In one embodiment, the reduced stress is achieved by employing a femoral slideway/femur-size template combination which results in installation of a femoral slideway which has a smaller dorsal-ventral dimension than a corresponding dimension of the femur prior to resection.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional of U.S. patent application Ser. No.09/517,674, filed on Mar. 2, 2000 now abandoned, the entire contents ofwhich is incorporated herein by reference. This Application also relatesto German Patent Applications DE 29906909.5, filed Apr. 16, 1999, and DE299 03 766.5, filed on Mar. 2, 1999.

BACKGROUND

1. Field of the Invention

The invention relates to a femoral slideway, and more specifically to afemoral slideway/femur-size template arrangement and a kneeendoprosthesis system with such a femoral slideway.

2. Description of the Related Art

A femoral slideway of this kind is disclosed, for example, in the Germanpatent DE 40 41 002 C2. In the surgical technique customarily used forknee-joint replacement by means of such a femoral slideway, an equalamount of bone is removed from the two condyles of the femur, so thatthe anterior or ventral cut is parallel to the posterior or dorsal cut.When the implant is in the position thus defined, the axis of rotationof the implant no longer coincides with the axis specified by thearrangement of the collateral ligaments, and this position is notanatomical inasmuch as when flexed, the implant is seated either tootightly on the medial side or too loosely on the lateral side.

The remedy that was recommended some time ago, namely an outwardrotation of the cutting guide such that in the posterior region lessbone is removed laterally than medially, whereas anterior-laterally morebone is removed than on the anterior-medial side, also presentsdisadvantages, which the construction of the femoral slideway proposedin DE 197 16 879 A1 of the applicant is designed to eliminate. The cruxof this solution is to rotate the anterior or ventral cut in thetransverse plane.

However, this more recent solution also requires improvement withrespect to optimizing the joint function in cooperation with thecollateral ligaments in particular, especially in order to reduce theload imposed thereon.

It is thus the object of the invention to disclose a femoral slidewaywith further improved function, as well as a knee endoprosthesis systemthat can be efficiently constructed and employed and has such a femoralslideway as its essential element, and finally an advantageousarrangement comprising femoral slideway and femur-size template.

This object is achieved in its first, foremost aspect by a femoralslideway with the features described in the present application.

The invention includes the essential idea that it is advantageous toprepare for a knee-joint replacement by resecting more bone from thefemur dorsally than is replaced by the implant (the femoral slideway).The “diminution” of the femoral slideway thus brought about in thedorsal region, in comparison to the original dimensions of the(resected) condyles or to a femoral slideway fitted in the conventionalmanner, produces an effective reduction of the turning radius of thetibial plate belonging to the prosthesis system and hence reduces theload on the collateral ligaments.

The decrease in the dimensions of the femoral slideway measured betweenthe outermost, dorsoventrally opposed points on the condyle shellsurfaces, in comparison to the previously customary dimensioning, ispreferably in the range between 2 and 5%. This is achieved byconstructing the associated femoral slideway/femur-size templatearrangement in such a way that the distance separating one or more pegson the femoral slideway from its dorsal sliding surface is smaller by5-15%, in particular by about 10%, than the corresponding distance bywhich bores in the femur-size template for positioning the pegs areseparated from the contact surface that is to be apposed to the dorsalcondyle surfaces of the femur.

The distance between the dorsal sliding surfaces and the one or morepegs on the inside of the femoral slideway is preferably in the rangebetween 24 and 34 mm and in particular is 29 mm, the chosen valueadvantageously being kept constant in a knee endoprosthesis system forcovering a relevant joint-size range.

As noted above, the femoral slideway/femur-size template arrangement isconstructed in such a way that the distance between one or more pegs onthe femoral slideway and its dorsal sliding surface is smaller than thecorresponding distance between bores in the femur-size template and thecontact surface of the template by 5-15%. and in particular about 10%.Stated differently, the corresponding distance between bores in thefemur-size template and the contact surface of the template is larger by5-15%, an in particular about 10%. than the distance between one or morepegs on the femoral slideway and its dorsal sliding surface.Accordingly, the distance between the bores in the femur-size templateand the contact surface of the template can be between 26.4 mm and 37.4mm (i.e., 10% larger than the range of 24 mm to 34 mm between the dorsalsliding surface and the one or more pegs of the femoral slideway). Inanother embodiment, the distance between the bores in the femur-sizetemplate and the contact surface of the template can be about 32 mm(i.e., about 10% larger than a distance of 29 mm between the dorsalsliding surface and the one or more pegs of the femoral slideway). Inyet another embodiment, the distance between the bores in the femur-sizetemplate and the contact surface of the template can be between 30.45 mmand 33.35 mm (i.e., 5%-15% larger than a distance of 29 mm between thedorsal sliding surface and the one or more pegs of the femoralslideway).

Another distinguishing feature of the proposed femoral slideway is thatparticular dimensions maintain a largely constant relationship to oneanother, regardless of the size of the actual prosthesis. For instance,the ratio a:c between the maximal dorsoventral extent and the maximallateral extent of the femoral slideway is about 0.9±0.02. The patellarpit formed between the condyle shells preferably has a depth b, measuredfrom the dorsalmost point on the condyle shells, such that its ratio b:ato the maximal dorsoventral extent of the femoral slideway is in therange between 0.4 and 0.5, in particular is 0.44.

The patellar pit is thus lengthened in the dorsal direction, as a resultof which the patella can be supported over a large area throughout itsentire functional range of flexion.

This elongation of the patellar pit, which furthermore increases inaccordance with the anatomy in implants of all sizes, allows for thefact that the patello-femoral contact surface in conventional femoralslideways has a relatively small bearing area. That is, in the region inwhich the patella leaves the trochlea and enters the intercondylarfossa, conventional femur components provide support only in theperipheral regions.

Furthermore, in the proposed femoral slideway the condyle shells aresomewhat more strongly rounded in cross section (coronal section) thanis the case in conventional femoral slideways. This modification wasundertaken in the interest of improving the fit to the special tibiainsert that belongs to a knee endoprosthesis system, but which is notwithin the scope of the invention.

The back surface of the femoral slideway, in one advantageousembodiment, bears a two-component Ti coating produced in a vacuum plasmaprocedure, consisting of a relatively thin, dense base layer and aseveralfold thicker, open-pored cover layer. The dense base layer allowsthe femoral slideway, which for example consists of CoCrMo, to becomecompletely sealed to the bone and, because it makes contact with thesubstrate over a large area, increases the stability of adhesion.

The open-pored and very rough surface of the cover layer provides idealconditions for the growth of bony substance onto and into the carriage,producing a quasi “3-D interlocking” that can transmit pulling forces aswell as pressure and transverse forces.

Additional advantages and useful features of the invention will beapparent from the subordinate claims and the following description of anexemplary embodiment with reference to the figures, wherein

BRIEF DESCRIPTION Of THE DRAWINGS

FIG. 1 is a view (from proximal) of a femoral slideway according to oneembodiment of the invention,

FIG. 2 shows the femoral slideway according to FIG. 1 in median section(sagittal section),

FIG. 3 is a plan view of an embodiment of a femur-size template,

FIG. 4 is a side view of the latter,

FIGS. 5 a, 5 b show a conventional arrangement of a femoral slideway ona femur in comparison to an arrangement proposed here, and

FIGS. 6 a, 6 b show scanning electron micrographs of a cross section ofa conventional layered structure and of an embodiment of the layeredstructure proposed here for the back-surface coating of a femoralslideway.

DETAILED DESCRIPTION Of The PREFERRED EMBODIMENTS

In FIGS. 1 and 2 the femur component 10, called a femoral slideway, of aknee endoprosthesis is shown. The femoral slideway 10 comprises twoconvexly curved condyle shells 11, 12 and a patellar shield 13, whichconnects the two condyle shells 11, 12 rigidly to one another.

The condyle shells 11, 12 and the patellar shield 13 in their interiorsdefine anterior and posterior fitting surfaces 14, 15 that correspond toa femoral ventral and dorsal cut, respectively, and are associated witha ventral and a dorsal saw-cut surface produced at the distal end of thefemur when the latter was resected for fitting of the femoral slideway.The convex outer shape of the condyle shells 11, 12 specifies dorsalsliding surfaces 11 a, 12 a in the posterior region, over which thecorresponding surfaces of the tibia insert slide when the kneeendoprosthesis is flexed. The patellar shield 13, which is recessed withrespect to the convex outer surfaces of the condyle shells 11, 12,defines a so-called patellar pit 16, within which there is supported apatella component 17 of the knee endoprosthesis, which is indicated by adashed outline in FIG. 2 and does not belong to the femoral slideway 10.

To assist anchoring and central placement of the femoral slideway 10 onthe femur, on the inner surface of the femoral slideway two pegs 18, 19are formed, the long axis of which is substantially parallel to theposterior fitting surface 15. These pegs project into holes in thefemur, which have been drilled in the appropriate positions with the aidof a corresponding drilling template (see below), and this engagementgives the attachment of femoral slideway to bone greater stability thanis provided by the fitting surfaces alone.

To ensure that the the femoral slideway will function optimally as areplacement for destroyed sliding surfaces on the femur, theconstruction must reflect as accurately as possible the anatomicalarrangements and dimensions, but also within the scope of the inventionincludes a specific modification that will now be explained.

One of the relevant dimensions of the femoral slideway 10 is the maximalanterior-posterior or dorsoventral extent of the condyle shells 1, 12, adistance labelled a in FIG. 1. Another relevant dimension is the maximallateral extent of the femoral slideway, i.e. the distance between themost lateral point on the lateral condyle shell 11 and the most medialpoint on the medial condyle shell 12, which in FIG. 1 is labelled c.Also significant is the distance from the outermost posterior point onthe dorsal sliding surfaces 11 a, 12 a of the condyle shells 11, 12 tothe posterior bounding edge of the patellar shield 13, which in FIG. 1is labelled b. A final significant distance is that between theoutermost posterior points on the dorsal sliding surfaces 11 a, 12 a andthe long axis of the pegs 18, 19 (which lie in one and the same coronalplane), labelled d in FIG. 2. In the exemplary embodiment describedhere, the ratio a:c is 0.9 and the ratio b:a is 0.44. On grounds ofbiomechanics and surgical technique, it has proved useful to make thedistance d (between sliding surface and peg axis) uniform for all sizesof femoral slideway used in a knee endoprosthesis system. In the presentcase, this distance is 29 mm.

To determine the correct femoral slideway size, a femur-size template 20shown in FIGS. 3 and 4 is used. This comprises a basic part 21 with twoflanks 22 and 23, each of which ends in a contact section 22 a, 23 athat is bent at a right angle and is apposed to the condyles of afemoral bone that is to be fitted with a femoral slideway (FIGS. 1 and2).

In the middle of the basic part 21 a measurement tongue 24, bent at anangle in two places, is mounted so that it can be displaced in adirection perpendicular to the plane in which the contact sections 22 a,23 a lie. The measurement tongue 24 is marked with a scale 25, whichindicates the maximal anterior-posterior extent of the head of thefemur, i.e. the condyles, and thus indicates to the doctor the requiredsize of the implant. In the basic part 21 of the femur-size template 20two peg-hole bores 26, 27 are provided, which—in accordance with asupplementary drilling-template function of the femur-sizetemplate—assist the positioning of peg-holes in the femur so that theycorrespond to the pegs 18, 19 of the femoral slideway 10 as shown inFIG. 1. The axes of the peg-hole bores 26, 27 are separated by adistance e from the contact surfaces of the contact sections 22 a, 23 a.This distance—along with the distance d between the sliding surfaces andpegs on the femoral slideway 10 itself (cf. FIG. 2)—is an additionalrelevant dimension in the concrete implementation of a kneeendoprosthesis, for the following reason:

So that the above-mentioned peg-holes—which serve not only to positionthe implant but also to position the cutting guides used to produce thevarious saw cuts on the femur—can be drilled into the bone, drill bushes(not shown) are inserted into the peg-hole bores 26, 27.

It has proved advantageous, in particular from the viewpoint of reducingthe load on the collateral ligaments during flexion of the artificialknee joint, to resect more bone dorsally on the femur than will bereplaced there by the thickness of the dorsal parts of the condyleshells. For this reason the distance e is made larger than thecorresponding distance d (FIG. 2). In the preferred embodiment therelative distance reduction, i.e. the quantity (e−d)/d, is about 10%.

The effect thus achieved can be seen in FIG. 5, where a sketchrepresenting the conventional way of attaching a femoral slideway 10′ toa femur F′, shown in FIG. 5 a, is compared with the representation inFIG. 5 b of the arrangement proposed here. The anterior-posterior extentof the femoral slideway 10 in FIG. 5 b, mounted on a femur F resectedfurther in the dorsal region, is smaller by the amount (e−d) than in theconventional implant 10′.

Because the distance e is permanently specified by the femur-sizetemplate, which is used for all implants regardless of their size, andaccording to what has been stated above the distance d in the embodimentof the femoral slideway is preferably kept constant for all implantsizes, the geometric relations will be slightly different for implantsof different sizes. This is acceptable, however, in view of theadvantages for manufacture and manipulation that such a system brings.

FIG. 6 b shows—in comparison to a conventional femur-carriage coating asshown in FIG. 6 a—the appearance in the scanning electron microscope ofa cross section through a two-component titanium-coating constructionconsisting of a dense base layer G, about 50 μm thick, and an open-poredcover layer D averaging about 250 μm thick, on a CoCrMo substrate S.Although the thickness and average roughness of the coating according toFIG. 6 b, which is applied by a vacuum plasma process, are comparable tothose of the known, sprayed-on coating according to FIG. 6 a, it shouldbe emphasized that the former has a more open-pored structure and aconsiderably reduced number of interface defects (indicated in bothpictures by vertical arrows).

Implementation of the invention is not limited to the exemplaryembodiment described above, but can also incorporate modifications,which in particular include departures from the specified dimensions andratio values.

LIST OF REFERENCE NUMERALS 10, 10′ Femoral slideway 11, 12 Condyleshells 11a, 12a Dorsal sliding surfaces 13 Patellar shield 14 Anteriorfitting surface 15 Posterior fitting surface 16 Patellar pit 17 Patellacomponent 18, 19 Peg 20 Femur-size template 21 Basic part 22, 23 Flanks22a, 23a Contact sections 24 Measurement tongue 25 Scale markings 26, 27Peg-hole bores a, b, c, d, e Distances A-A Plane of section D Coverlayer F, F′ Femur (shaped) G Base layer S Substrate

1. A method of preparing a lower extremity of a femur and implanting a femoral slideway thereon, the method comprising: providing a femoral slideway having at least one peg adapted for insertion into a hole in the femur; locating a point on a lower extremity of a femur, said point being located at a pre-determined distance from a plane tangent to dorsalmost points of lateral and medial condyles of the femur, the pre-determined distance defined by a template having a permanently specified distance between a bore on a support part and a contact surface on a flank of the template, the support part and flank defined as a single piece, the contact surface configured to engage at least one of the dorsalmost points of the lateral and medial condyles of the femur; drilling a hole at said point; and selecting a femoral slideway having a dimension defined by a perpendicular distance between a longitudinal axis of a peg extending from the slideway and a plane tangent to a dorsal sliding surface farthest away from the peg; wherein said pre-determined distance is larger than said dimension.
 2. The method of claim 1, further comprising resecting bone material from the femur and fitting the slideway onto the femur by inserting the peg into the hole.
 3. The method of claim 1, wherein said pre-determined distance is about 5% to about 15% larger than said dimension.
 4. The method of claim 3, wherein said pre-determined distance is about 10% larger than said dimension.
 5. The method of claim 1, wherein said locating is performed before said drilling.
 6. The method of claim 1, wherein said selecting a femoral slideway further comprises selecting a slideway with a dimension between a dorsal sliding surface and a ventral sliding surface which is a previously-determined percent of a distance between a dorsal-most condylar surface and a ventral-most condylar surface of the femur before said resecting.
 7. The method of claim 6, wherein said percent is about 2 to 5%.
 8. The method of claim 1, wherein selecting further comprises selecting a femoral slideway from a group of slideways, wherein all of the slideways in said group have substantially equal peg-to-sliding-surface dimensions as defined by a perpendicular distance between a longitudinal axis of a peg extending from the slideway and a plane tangent to a dorsal sliding surface furthest away from the peg; and wherein each slideway in said group has a different dorsal condyle-to-ventral condyle distance.
 9. The method of claim 1, wherein the bore and the contact surface are defined on a unitary part of the template.
 10. A method of preparing a lower extremity of a femur for implantation of a femoral slideway, said method comprising: determining a location for a hole to be drilled in a lower surface of a femur bone by indicating a point at a previously-determined distance from a dorsal-most point of a condyle of said femur; wherein said previously-determined distance is 5 to 15% larger than a distance between a peg and a dorsal sliding surface of the femoral slideway to be implanted on said femur bone; drilling the hole in a lower surface of a femur bone at said point; resecting bone material from said femur, wherein said resecting includes removing more bone material from a dorsal side of the femur than will be replaced by the slideway to allow for a reduction of the distance between the peg and a center of the turning radius of the dorsal portion of the slideway compared to the unresected femur to reduce a load on ligaments attached to the femur; and implanting the slideway on the femur by inserting the peg into the hole.
 11. The method of claim 10, wherein said previously-determined distance is about 10% larger than a distance between a peg and a dorsal sliding surface of the femoral slideway to be implanted on said femur bone.
 12. The method of claim 11, wherein the distance between the peg and the dorsal sliding surface of the femoral slideway is between about 24 mm and about 34 mm.
 13. The method of claim 12, wherein the distance between a peg and a dorsal sliding surface of the femoral slideway is about 29 mm.
 14. The method of claim 11, wherein said previously-determined distance is between 26.4 mm and 37.4 mm.
 15. The method of claim 14, wherein the previously-determined distance is about 32 mm.
 16. The method of claim 10, wherein said previously-determined distance is between 30.45 mm and 33.35 mm.
 17. The method of claim 10, wherein resecting bone material from the dorsal side of the femur includes resecting an equal amount of bone from the medial and lateral posterior condyles.
 18. A method of implanting a femoral slideway on a femur, said method comprising: providing a slideway comprising two convexly curved condyle shells rigidly connected to one another anteriorly by a patellar shield, wherein outer surfaces of the condyle shells define dorsal sliding surfaces of the femoral slideway, wherein the femoral slideway has on an inner surface at least one peg, wherein between a long axis of the peg or pegs and a point on a dorsal sliding surface furthest away therefrom a sliding-surface-to-peg distance is defined, and wherein an overall extent of the femoral slideway in an anterior-posterior direction is smaller than a corresponding original overall extent of the condyles of a femur to which the femoral slideway is fitted; resecting bone material from a femur, wherein said resecting includes removing more bone material from a dorsal side of the femur than is replaced by the slideway; providing at least one hole in said femur to receive said at least one peg of said slideway and to secure and locate said slideway on said femur, the position of said at least one hole being a pre-determined distance from a plane tangent to dorsalmost points of lateral and medial condyles of the femur, the pre-determined distance defined by a femur-size template having a permanently specified distance between a bore on a support part and a contact surface on a flank of the template, the support part and the flank defined as a single piece, the contact surface configured to engage at least one of the dorsalmost points of the lateral and medial condyles of the femur; and fitting said slideway onto said femur.
 19. The method of claim 18, wherein said resecting includes removing about 10% more bone material from the dorsal side of the femur than is replaced by the slideway, as measured linearly between said holes and a dorsalmost surface of said femur.
 20. The method of claim 18, further comprising positioning the femur-size template on said femur to determine the position of said at least one hole in the femur.
 21. The method of claim 20, wherein said femur-size template comprises a dorsal condyle-engaging surface and at least one bore separated by a pre-determined distance.
 22. The method of claim 21, wherein said pre-determined distance is about 5 to 15% larger than a perpendicular distance between said at least one peg and said dorsal sliding surfaces.
 23. The method of claim 22, wherein said pre-determined distance is about 10% larger than a perpendicular distance between said at least one peg and said dorsal sliding surfaces.
 24. The method of claim 18, wherein resecting bone material from the dorsal side of the femur includes resecting an equal amount of bone from the medial and lateral posterior condyles.
 25. A method of preparing a lower extremity of a femur and implanting a femoral slideway thereon, the method comprising: providing a femoral slideway having at least one peg adapted for insertion into a hole in the femur; locating a point on a lower extremity of a femur, said point being located at a pre-determined distance from a plane tangent to dorsalmost points of lateral and medial condyles of the femur, the pre-determined distance defined by a template having a permanently specified distance between a bore on a support part and a contact surface on a flank of the template, the support part and flank permanently connected to each other, the contact surface configured to engage at least one of the dorsalmost points of the lateral and medial condyles of the femur; drilling a hole at said point; and selecting a femoral slideway having a dimension defined by a perpendicular distance between a longitudinal axis of a peg extending from the slideway and a plane tangent to a dorsal sliding surface furthest away from the peg; wherein said pre-determined distance is larger than said dimension.
 26. The method of claim 25, wherein the previously-determined distance is 5 to 15% larger than the distance between the peg and the dorsal sliding surface of the femoral slideway to be implanted on the femur bone. 